1. Field
The present invention relates to a direct-current switch suitable for making a direct-current path, along which a direct current flows, an open circuit or a closed circuit.
2. Description of the Related Art
To date, alternating-current power has been supplied to general households from an alternating-current utility grid (a commercial power supply) using a synchronous generator. Meanwhile, in recent years, dispersed power sources using photovoltaic power generation, wind power generation, fuel cell power generation, or the like, have attracted attention, and have started to be used in general households. It is often the case that power generated by these dispersed power sources is direct-current power. A direct-current power supply that supplies the aforementioned power from a dispersed power source directly to a general household, office, or the like, is becoming accepted by society.
When supplying direct-current power from a utility grid (a direct-current power source) to a direct-current distribution system (for example, to indoor wiring that carries direct-current power), and using the power, it is necessary to interpose a direct-current switch between the indoor wiring and an electrical instrument (for example, a television receiver), and control whether or not to supply power to the electrical instrument. Herein, characteristics required of the direct-current switch (a switch carrying out an establishment of continuity and a shutting-off of direct-current power) differ greatly from characteristics required of a heretofore known alternating-current switch (a switch carrying out an establishment of continuity and a shutting-off of alternating-current power). The heretofore known alternating-current switch is standardized based on the turning on and off of an electric light illuminated by alternating current. To date, various miniature types have been widely used as the aforementioned alternating-current switch. However, when using this kind of miniature alternating-current switch in “a current path along which a direct current flows” (hereafter referred to as a direct-current path), the amount of current which can be shut off is limited to an extremely small amount. The reason for this is that, unlike with alternating current, there is no time at which direct current becomes zero, meaning that an arc generated when the mechanical contacts of the switch open continues to be generated continuously and without stopping, and an arc current caused by generation of the arc continues to flow. Then, on an arc being once generated, the arc current continues to flow, and it may happen that it is substantially not possible to put the mechanical contacts into an opened condition (a condition in which the switch is shut off). Also, it may happen that a burnout of the contacts occurs due to the heat generated by the arc. Then, a switch that can withstand the heat generated by the arc and enable the contacts to be opened is extremely large. That is, the heretofore known alternating-current switch is not suitable for use in an electrical instrument (for example, a household electrical product) that operates on direct-current power supplied from a direct-current power source.
Therefore, a direct-current switch shown as the related art in FIG. 14 has been proposed (refer to JP-A-2007-213842). The direct-current switch shown in FIG. 14 is suitable for use in a direct-current distribution system 110. A direct-current switch 120a has an input terminal A, an input terminal B, an output terminal C, and an output terminal D. The direct-current switch 120a includes a mechanical open/close switch 116, an electronic open/close switch 115, a switch control circuit 114 that controls the opening or closing time difference mutually between the mechanical open/close switch 116 and the electronic open/close switch 115, and a control switch 117. Then, the mechanical open/close switch 116 is opened after the electronic open/close switch 115 inserted in series in a bus bar 13 has been opened. By so doing, an arc is prevented from being generated in a condition in which the mechanical open/close switch 116 is opened (the current path is shut off), and it is possible to shut off (open) the current path of direct-current power supplied to a load 130 with a miniature mechanical open/close switch 116.
In the direct-current switch 120a disclosed in JP-A-2007-213842, continuity is established in both the mechanical open/close switch 116 and the electronic open/close switch 115 when establishing continuity (closing) of the direct-current path. Herein, it may be that although the contact resistance of the mechanical open/close switch 116 is in the region of, for example, a few mΩ (milliohm), the contact resistance of the electronic open/close switch 115 is in the region of, for example, a few hundred mΩ. For this reason, when the aforementioned direct-current switch establishes continuity (closing) of the current path for a long time, resistance loss (power loss) in the electronic open/close switch 115 cannot be ignored, and heat generation due to the resistance loss cannot be ignored either.
Herein, in order to reduce the contact resistance of the electronic open/close switch 115, a possible solution is to increase the chip size of the electronic open/close switch 115, which is formed from a semiconductor, and reduce the resistance when continuity is established. Also, a possible solution is to reduce the turn-on voltage when continuity is established. Furthermore, with regard to heat generation occurring in the electronic open/close switch 115, while it is not possible to prevent the heat generation itself, it is possible to prevent a rise in temperature of the electronic open/close switch 115 by using a heat sink formed from a material with a high thermal conductivity. However, when increasing the chip size, the cost of the electronic open/close switch 115 increases. Also, when using a heat sink, it is not possible to avoid an increase in size of the direct-current switch.